Antibodies play a central role in defense against invading non-self molecules. Antibodies' ability to interact with neonatal Fc-receptor (FcRn) in a pH-dependent manner confers them with extended serum half-life (Ghetie and Ward 2000). This unique feature of antibodies allows extending the half-life of therapeutic protein or peptide in the serum by engineering Fc-fusion molecules. Naturally occurring IgG antibodies and the engineered Fc-fusion molecules are bivalent and monospecific. This is due to the homodimeric nature of the Fc. For certain therapeutic applications, it would be desirable to retain all the positive attributes conferred by the antibody or the Fc fragment of the antibody, but achieve monovalent specificity by engineering monomeric Fc.
Antibodies belong to the immunoglobulin class of proteins which includes IgG, IgA, IgE, IgM, and IgD. The most abundant immunoglobulin class in human serum is IgG whose schematic structure is shown in the FIG. 1 (Deisenhofer 1981; Huber 1984; Roux 1999). The IgG structure has four chains, two light and two heavy chains; each light chain has two domains and each heavy chain has four domains. The antigen binding site is located in the Fab region (Fragment antigen binding) which contains a variable light (VL) and a variable heavy (VH) chain domain as well as constant light (LC) and constant heavy (CH1) chain domains. The Fc (Fragment crystallizable) fragment of the antibody contains CH2 and CH3 domain region of the heavy chain. The IgG molecule can be considered as a heterotetramer having two heavy chains that are held together by disulfide bonds (—S—S—) at the hinge region and two light chains. The number of hinge disulfide bonds varies among the immunoglobulin subclasses (Papadea and Check 1989). The FcRn binding site is located in the Fc region of the antibody (Martin, West et al. 2001), and thus the extended serum half-life property of the antibody is retained in the Fc fragment. The Fc region alone can be thought of as a homodimer of heavy chains comprising CH2 and CH3 domains.